Conventionally, optical disks such as a CD (compact disk) and DVD (digital versatile disk) are used as a medium for recording music data, video data, etc. An optical disk device condenses a laser light by a lens and irradiates it onto a data recording layer of an optical disk and, by detecting the light reflected thereby, reads out recorded data of the optical disk. An infrared laser light of a wavelength on the order of 780 nm and an objective lens with a numerical aperture on the order of 0.45 to 0.51 are used in reading out the recorded data of the CD.
On the other hand, with respect to the DVD, since the data is recorded at the density higher than that of the CD, a red laser light of shorter wavelength, for example, on the order of 660 nm and the objective lens with numerical aperture on the order of 0.65 are used. The CD and the DVD differ in depth from the surface on the side of the disk on which the laser light is irradiated to a data recording layer. Therefore, the optical disk device having compatibility for both of the CD and the DVD is required to have laser light sources of different wavelengths and different optical systems such as objective lens, for the CD and the DVD, respectively.
Ordinarily, however, a common optical system such as objective lens is used for the CD and the DVD, with a view to cost reduction and miniaturization of the optical disk device. For this reason, the optical system optimized for the CD (for the DVD) produces aberration for the DVD (or the CD). Therefore, the optical disk device having compatibility for the CD and the DVD is required to correct this aberration.
A mass-storage DVD having two data recording layers is on the market. With such a DVD, since the depth from the surface on the side of the disk on which the laser light is irradiated to the data recording layer is different between the data recording layers, there is no problem with the data recording layer for which the optical system is optimized, but spherical aberration is produced with respect to other data recording layer. Even with an ordinary single-layer optical disk such as the DVD and the CD, if the thickness of the disk varies, the depth from the surface on the side of the disk on which the laser light is irradiated to the data recording layer varies and the spherical aberration is produced.
In recent years, in accordance with development of a blue semiconductor laser, a next-generation optical disk and optical disk device are developed that has the recording density further enhanced by use of a blue laser light. However, since the spherical aberration is inversely proportional to the wavelength of the laser light, how to correct the spherical aberration becomes an important problem for such a next-generation optical disk device.
In the case of using the blue laser light, since the numerical aperture of the objective lens is 0.85, the effective aperture of the objective lens becomes large, as compared with the case of conventional CD and DVD using the infrared or red laser light. The position of the peak of the produced spherical aberration in the direction of the lens radius differs between the case of using the blue laser light and the case of using the infrared laser light or the red laser light. This situation is shown in FIG. 20. In FIG. 20, a horizontal axis and a vertical axis represent a distance (normalized) from the center of the objective lens in its radius direction and a produced spherical aberration volume (normalized), respectively. W-shaped curves in a solid line and a dashed line represent two-dimensionally the aberration in the case of using the blue laser light and the aberration in the case of using the red laser light, respectively.
As is apparent from FIG. 20, the effective aperture of the objective lens and the position of the peak of the produced aberration largely differ between the case of using the blue laser light and the case of using the red laser light. The case is the same when the produced aberration takes an M-shape curve, with the W-shape curve of FIG. 20 being reversed around the horizontal axis. Therefore, to ensure compatibility among the cases of using the blue laser light, the infrared laser light, and the red laser light with a common configuration of the optical system such as the objective lens, it is also an important problem how to solve the difference in the numerical aperture and the effective aperture, and the difference in the peak position of the spherical aberration, which depend on the difference in the wavelength of the laser light used.
Publicly known is a focusing mechanism that changes a focal distance or focal position of the optical system, having a liquid crystal layer, transparent substrates that sandwich the liquid crystal layer, a voltage drop resistor provided on a first of the transparent substrates from its center toward its periphery, a plurality of concentric electrodes provided on the first of the transparent substrates and connected to the voltage drop resistor, extraction electrodes for applying voltage across the voltage drop resistor, a lower electrode provided on a second of the transparent substrates for applying an electric field to the liquid crystal layer between itself and the concentric electrodes, and a power source for applying voltage to the extraction electrode and the lower electrode (see, for example, Patent Reference 1). According to the Patent Reference 1, by appropriately determining the width of the voltage drop resistor, the focal position at which the image is formed by the focusing mechanism can be made variable.
Publicly known is an optical pickup for reading out data from the DVD having a plurality of data recording layers, including a wave front aberration correcting unit located in a light path between a light source and an objective lens that corrects wave front aberration of the light projected from the light source dependent on the data recording layer selected, the wave front aberration correcting unit being composed of a liquid crystal element (see, for example, Patent Reference 2). According to the Patent Reference 2, correction can be made of the wave front aberration produced by a difference in distance between the recording layer for which the optical pickup is optimally designed and each of the other recording layers.
Patent Reference 1: Japanese Patent Publication No. 3047082
Patent Reference 2: Japanese Patent Application Laid-Open Publication No. 1998-269611